By Edith Honan

NEW YORK (Reuters) - Gay rights activists are looking to a possible vote to legalize same-sex marriage in New York State on Tuesday as a way to drive a national campaign that opponents say has lost momentum.

Five U.S. states have already legalized gay marriage -- Iowa, Connecticut, Massachusetts, New Hampshire and Vermont -- but earlier this month voters in a sixth, Maine, chose to repeal such a law, which was due to take effect in September.

Forty U.S. states have laws banning gay marriage.

New York's Democratic-controlled State Assembly passed a bill legalizing same-sex marriage in May, but the proposed legislation faces a tougher battle in the Senate, where the party has a slim majority.

Governor David Paterson, a Democrat who supports gay marriage, has urged the upper chamber to pass the bill and vowed to sign it into law.

New York is one of the most politically liberal states in the nation, but it is also home to large numbers of Catholics and African-Americans, many of whom oppose gay marriage.

Two recent polls showed a majority of New York voters in favor of allowing same-sex couples to marry, but one other poll showed the public evenly split.

"Winning gay marriage in New York will be a boost for gay marriage across the country," said Alan Van Capelle, the executive director of the Empire State Pride Agenda, a group that is lobbying for the legislation.

"I am optimistic that this will come to the floor for a vote (on Tuesday) and that it will pass," he said.

But Maggie Gallagher, the leader of the anti-gay marriage group, National Organization for Marriage, said she did not believe the bill would earn the needed Republican support.

"I don't think it was ever true that the culture had shifted on the gay marriage question," she said. "To the voters, this was very strange. The politicians were obviously listening to something other than the people's priorities."

Some observers say the battle may have been complicated by a New York congressional election this month that saw a gay marriage opponent and third party candidate force pro-gay marriage Republican Dede Scozzafava out of the race.

Although the Democratic candidate, Bill Owens, won the election, the surge in support for conservative Douglas Hoffman may make moderate Republicans think twice about supporting the proposed bill in New York.

"I think Republicans who might have been supportive in the past might have been spooked by the Owens-Scozzafava-Hoffman race," Malcolm Smith, a Democrat and the Senate president, told the New York Times.

'PROGRESSIVE STATE'

Gay marriage is also on the agenda in neighboring New Jersey, where Democratic Governor Jon Corzine has pledged to sign a bill into law if it is passed by the legislature before his term ends in January. Corzine lost re-election this month to anti-gay marriage Republican Chris Christie

But it remains uncertain if lawmakers in New Jersey, which already permits same-sex civil unions, will bring the issue to a vote. Civil unions afford couples largely the same rights as those of married couples -- from insurance coverage to tax benefits and hospital visiting rights.

Steven Goldstein, who heads Garden State Equality in New Jersey, a pro-gay marriage group, said objected to suggestions that the Maine referendum or the outcome in New York would dictate what happens in his state.

"You could not pick two states more different than Maine and New Jersey," he said. "We follow New Jersey values and New Jersey is a progressive state."

U.S. President Barack Obama recently won a standing ovation at a dinner held by Human Rights Campaign, a gay advocacy group, when he said he would fight for their causes and renewed a pledge to end restrictions on gay service in the military.

But many gay activists complain that Obama has done little to back up his rhetoric on gay rights, including not repealing the Defense of Marriage Act, which bars the federal government from forcing states to recognize gay marriage.

"There's tremendous frustration with the White House," said Jeffrey Campagna, a political fundraiser and the founder of the New York gay-rights group The Power. "It's time for this inspirational political figure to speak out on behalf of civil rights."

(Editing by Michelle Nichols and Paul Simao)